KATOOM

Does anyone know what kind of vacumm the vacumm pump is supposed to pull on these motors? What is the tell tale sign of a bad vacumm pump? Is there a code for that. I've heard of one's that leak but I didnt know if they go bad slowly or just stop sucking. Thanks for any info.

Haulin_in_Dixie

A good leak will also cause blowby as the exhaust for the pump is in the crankcase. This information came from Cummins, we tested mine and it was leaking some into the crankcase. To check that you put a cover over the gearcase and check the blowby without it. Cummins supplied the cover, bolts, and a 1/4 thick gasket that they use for testing engines. A leaky vacuum hose will cause the same thing with the blowby. MIne has been leaking a little oil for 150,000 miles but does not get any worse. Lately it looks like I will have to do something with it.

KATOOM

I dont understand what you mean. Blowby? Does that mean more than usual? Cover for crankcase? Where? Sorry I dont mean to be stupid but could you explain better? Thanks.

Haulin_in_Dixie

Blowby is the vapor coming out of the breather tube at the front of the engine. If it is excessive it is usually from warn rings, but in this case it can be caused by the vacuum pump.

thumper 549

you can only pull about 1 atmosphere somewhere around 14 to 18 in

KATOOM

I understand the blowby but I dont understand why a bad vacumm pump would cause more. You said the pump exhaust is in the crank case. That tells me that there is air being pumped into the crank case by the vacumm pump all the time. So why would a bad vacumm pump be the culprit for exessive blowby? I would think lack of blowby would mean bad vacumm pump. Same thing for a bad vacumm hose. Maybe I dont understand how the whole thing works. I do understand worn rings causing exessive blowby though.

Where did you get that information? Is that at idle or another RPM?

Haulin_in_Dixie

I will try to explain it, might not do so good. If you suck on a straw, the soda comes up through it. If you seal the end of the straw not much of anything comes through. Everyting that is operated by vacuum is sealed. It might pull on a diaphram, but as soon as it is pulled, it has no more air to draw. So nothing much goes through the system. If you have a leak in the system or a seal is bad on the pump, then it is pumping air into the crankcase, coming out as blowby. If everyting is working right there is little to nothing coming into the crankcase.

If you remove the bolts that hold the vacuum pump into the gearcase there is a large hole. The cover that Cummins supplied covers this hole.